In most cases, their evil motivation is jealousy. The Queen in Snow White and Ursula the scheming meroctopus from The Little Mermaid are overwhelmed with rage about being alone and unattractive. Even though the Queen is, according to the magic mirror, the second most attractive woman on Earth, she's still twisted into insanity over only winning silver. In her case, as well as that of the evil witch from Tangled, their bitterness eventually winds up bringing out their true forms, which are of course broken-down, ancient hags. You know, the worst possible thing you can ever be.
Girls, you better put a ring on that finger before you wind up spending your days weeping over a crystal ball in some decrepit castle spire.
Walt Disney Pictures
This was a single's bar until the clock struck midnight on her 30th birthday.
In the real world, of course, single people are doing fine. Even though popular opinion mirrors Disney in believing that reaching 30 without getting married reflects failure and will result in a bitter depression, studies show that single adults are generally no less happy than married people, and happier than those divorced or widowed. In fact, autonomy (the ability to do whatever you damn well please, whenever you want) is one of the best indicators of overall life satisfaction. Aging isn't a huge deal either, since we are seeing that people in general get happier as they get older. So really, making it past thirty without a ball and chain allows you to have a super awesome, happy life where you don't have to convince anyone else that popcorn, booze, and frosting counts as dinner.